Biology and Control
of Major Pacific Northwest Grape Pests
Insect pests, weeds, and especially diseases can adversely affect the quantity and quality of wine and juice grapes produced in the Pacific Northwest. This section will discuss briefly the damage caused, life cycle, and control of important wine and juice grape pests. The pests included are ones for which pesticides were applied on at least 10% of the acres in one or more of the four industry segments (Idaho wine grapes, Oregon wine grapes, Washington wine grapes or juice grapes).
The exception is black vine weevil; less than 1% of juice grape acres were treated per year on average for this pest. However, if the only effective registered insecticide that controls this pest, carbofuran, were lost, black vine weevil could become a very serious pest of both wine and juice grapes in the Pacific Northwest.
Insects
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Diseases
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Weeds
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Leafhopper (Idaho and Washington wine grapes)
Damage. Two species of leafhopper, the western grape leafhopper, Erythroneura elegantula, and the Virginia creeper leafhopper, Erythroneura ziczac, feed on grape leaves by puncturing the leaf cells and sucking out the cell contents. Repeated feeding causes reduced photosynthesis; heavily damaged leaves will desiccate and abscise. Thick infestations of leafhopper can defoliate a vineyard if no treatment is applied. Large populations of adult leafhoppers may be present in vineyards before harvest. These winged adults annoy workers by flying in their faces. Nymphs also cause worker discomfort by trying to pierce exposed skin.
Life cycle. Two generations of leafhopper occur per year in the Pacific Northwest. They overwinter as adults under weeds, fallen leaves, and other debris in the vineyard, and become active in March or April. They feed on weeds and other suitable plants until green leaves emerge on the grapevines, then lay eggs on the undersides of the leaves. The eggs hatch from mid-May to late June. Nymphs pass through five nymphal instar stages, during which they cannot fly, before becoming winged adults. First generation adults are active by mid-June, and second generation eggs are laid in early July. Second generation adults are active through harvest until late fall.
Control. Insecticides are most effective when the majority of
the leafhopper population consists of third and fourth instar nymphs. Eggs
are protected from exposure to insecticides because they are embedded in
the leaf tissue, and adults are more tolerant of insecticides than nymphs
are. When insecticide applications are properly timed, one or two treatments
should provide sufficient control for the season, depending on the material
used. One treatment of imidacloprid or dimethoate will usually give season-long
control, while two treatments of carbaryl, endosulfan or azinphos-methyl
may be necessary in high-pressure situations.
Thrips (Idaho and Washington wine grapes)
Damage. Various species of thrips can damage wine grape shoots, leaves, and fruit. The two species most commonly found on Pacific Northwest grapes are western flower thrips, Frankliniella occidentalis; and grape thrips, Drepanothrips reuteri. Onion thrips, Thrips tabaci; Thrips minuta; and Frankliniella minuta also appear occasionally, but are not damaging. During and shortly after bloom, thrips may scar the berries. As the berries enlarge, the scars restrict growth of the epidermis, producing misshapen and split berries. Extensive berry scarring can also lead to a severe loss of pigment in red varieties. Thrips feeding on shoots can severely stunt leaf and shoot growth in the spring and summer.
Life Cycle. Thrips overwinter as mature females under debris on the vineyard floor. In the spring they lay their eggs in flower, leaf, and stem tissue. Thrips have several generations per year; the precise number of generations varies among species.
Control. The same insecticides are effective against all species of thrips. Dimethoate, carbaryl, endosulfan, and azinphos-methyl are all commonly used for thrips control in grapes. Monitoring for thrips is done from budbreak through August. Treatments are applied when thrips numbers reach potentially damaging levels.
Cutworms (Washington wine grapes and juice grapes)
Damage. Two species of cutworm, the spotted cutworm, Amathes c-nigrum, and the redback cutworm, Euxoa ochrogaster, feed on developing buds of Washington wine and juice grapes in the early spring. Cutworm infestations occur in localized areas of vineyards. If a cutworm-infested area is not treated, yield loss could be 50% to 100%. Cutworms can severely weaken newly planted vines.
Life Cycle. Spotted cutworms overwinter as second or third instar larvae in the soil or under debris. They feed on winter annual weeds during warm periods in late winter and early spring. Beginning when the grape buds start to swell, spotted cutworms climb the vines at night to feed on buds and young shoots. During the day they remain under debris on the ground or under bark on the trunk. Redback cutworms overwinter as eggs that hatch about the time of budbreak. Larvae climb the vines and feed on buds and shoots at night. Redback cutworms normally stay under the bark during the day.
Control. Insecticides are applied when bud injury reaches 5% to 10% of total buds. Portions of vineyards that have historically had cutworm problems are often given preventive treatments. Cutworm control is especially critical in newly planted vineyards. Chlorpyrifos is the most effective insecticide for cutworm, but carbaryl and methomyl are also used.
Grape Mealybug (Idaho wine grapes, Washington
wine and juice grapes)
Damage. Mealybug, Pseudococcus maritimus, is a serious pest of both wine and juice grapes. This pest produces a honeydew that makes the fruit, shoots, and foliage sticky. A black fungus,Cladosporium spp., grows on the honeydew, producing a sooty mold. Serious honeydew and sooty mold contamination will make the fruit unsuitable for processing into wine or juice.
Life Cycle. Mealybugs overwinter as eggs and crawlers, usually under bark on the trunk and cordons. After budbreak, the crawlers feed on the new shoots. They mature in early summer, and the adults move back to the trunks and laterals to lay eggs. The second-generation crawlers move back to the green tissue, including the fruit, where they mature by late summer. The honeydew produced by the second generation can cause processors to reject the fruit.
Control. Insecticides are most effective when the pest is in the crawler stage; time treatments to coincide with this stage. A delayed dormant application of superior-type oil tank mixed with chlorpyrifos is most effective. A summer application of diazinon is sometimes applied.
Cottony Maple Scale (Idaho wine grapes, Washington
wine and juice grapes)
Damage. Cottony maple scale, Pulvinaria vitis, is a minor pest of both wine and juice grapes. Scale crawlers produce honeydew that causes the leaves and fruit to be sticky. A black fungus, Cladosporium spp., grows on the honeydew, producing a sooty mold. Honeydew and sooty mold may make the fruit unsuitable for processing into wine or juice.
Life Cycle. This pest overwinters as an immature scale. It matures in early summer and lays eggs. The crawlers become active in July and August.
Control. A dormant or delayed dormant application of a superior-type oil is the most effective treatment for scale. The crawlers also may be treated with diazinon in July.
Black Vine Weevil ( juice grapes)
Damage. Black vine weevil, Otiorhynchus sulcatus, is normally a problem only in vineyards more than 10 years old. This pest is mainly treated in juice grapes, but some wine grape vineyards have been treated. As the Pacific Northwest wine industry ages, black vine weevil problems will probably increase in wine grape vineyards. The adult weevils feed on grape clusters and leaves in early and mid-summer, girdling the cluster stems and berry stems. Damaged berries do not size or ripen properly and may abscise. This can result in a yield reduction of several tons per acre.
Life Cycle. The weevils overwinter as larvae, feeding on grapevine roots. Pupae form at some time in mid-April and remain 3 to 4 inches below the ground. Adults begin emerging around mid-May; emergence peaks about mid-June. These adults lay eggs about 3 weeks after emergence.
Control. Carbofuran is the only currently registered insecticide that controls black vine weevil. Carbofuran should be applied 3 weeks after the first adult weevils emerge; it is normally applied in early to mid-June. After an area is treated, the black vine weevil population takes about 5 years to build back up to economically damaging levels. Because weevils are only active at night and hide under vineyard floor debris and loose bark during the day, weevil populations may go undetected in a vineyard for a long time.
Powdery Mildew (Idaho, Oregon, and Washington wine grapes)
Damage. Powdery mildew, caused by the fungus Uncinula necator, is the most important disease of Pacific Northwest wine grapes. Juice grapes are normally thought to be resistant, but some juice grape growers had powdery mildew in their vineyards in 1995. This may indicate that juice grapes will be treated for this disease in the future.
All green tissues (leaves, fruit, and shoots) are susceptible to powdery mildew infection. The fungus forms a white, powdery layer of mycelia (tubal filaments) and spores over the surface of the infected tissue. Powdery mildew infection on the leaves reduces photosynthesis, and infection of the fruit stunts berry growth.
When more than 5% of grapes are mildew-infected, the wine made from the fruit can have a distinct off-flavor. Most wineries will reject mildew-infected fruit. If mildew is present on the rachis (stem to which berries are attached), the berries will fall off ahead of the grape harvester, causing yield loss.
Life Cycle. Uncinula necator overwinters as cleistothecia (spherical, black spore-containing bodies) on bark, fallen leaves, and other surfaces. The cleistothecia contain spores that are released following spring rains. These spores function as primary inoculum. They produce mycelia and secondary spores. The secondary spores are dispersed by wind to other parts of the vine, where they initiate secondary infections. These produce more mycelia and secondary spores. This cycle repeats itself many times throughout the growing season.
Control. A preventive fungicide program is essential for controlling powdery mildew. Four sterol-inhibiting fungicides, fenarimol, myclobutanil, triadimefon, and triflumizole, are registered for powdery mildew control.
Alternate or tank mix these fungicides with a sulfur product to slow the development of resistance to the sterol inhibitors. Begin sulfur treatments at 1 inch of shoot growth (April), and sterol inhibitors may be used starting at 6 to 12 inches of shoot growth (May). Continue treatments until veraison (the onset of ripening, which occurs in early August).
At veraison, the fruit is no longer susceptible to powdery mildew infection; treatments with powdery mildew fungicides are usually discontinued. Stems and leaves are still susceptible, however, so growers may choose in high disease pressure situations to apply one or two sprays after veraison.
Canopy management is an important nonchemical method of reducing mildew infection. Open canopies facilitate air movement, reduce shading, and allow better spray coverage, all of which reduce powdery mildew pressure. However, canopy management is not a substitute for well-timed preventive fungicide applications.
Bunch Rot (Oregon wine grapes)
Damage. Bunch rot is caused mainly by Botrytis cinerea, but many other secondary fungi and bacteria may be present in rotted clusters. Botrytis bunch rot causes the fruit to turn brown or dark purple, with fuzzy gray mycelia and spores visible on the cluster surface. Botrytis-infected clusters are vulnerable to infection by secondary rot fungi, including Aspergillus niger and Penicillium expansum. Bacteria, Acetobacter spp., and yeasts, Candida spp., also can invade the infected clusters, causing a vinegar-smelling "sour rot."
Bunch rot-infected fruit will be rejected by most wineries. If the rot is "clean" -meaning that it consists primarily of Botrytis cinerea without secondary rot organisms-it may be suitable for making late harvest dessert wine. However, only a limited number of tons of grapes are made into late harvest wines.
Life Cycle. Botrytis cinerea overwinters as sclerotia (black or reddish brown masses of threads) on or in colonized plant tissue, especially in grape mummy clusters from the previous year that have been left in the vineyard. Spring rains cause the sclerotia to release spores that are then spread by wind. When free water is available from rain or irrigation, the spores will germinate and infect shoots, leaves, and flowers. As the berries accumulate sugar, they also become susceptible to botrytis infection.
Control. Iprodione is the only fungicide widely used to control bunch rot. Many growers, however, believe it is ineffective. Fortunately, several management practices will reduce the incidence and severity of bunch rot.
Avoid overhead sprinkler irrigation after veraison if possible. Remove all of the previous season's clusters from the vines and the vineyard floor to reduce inoculum. Managing the canopy to maximize light and air penetration will create an environment less conducive to bunch rot development. It also will improve fungicide spray penetration. In years when rainy weather occurs during harvest, however, bunch rot may develop despite these precautions.
Annual Weeds (Idaho, Oregon, and Washington wine grapes, juice grapes)
Damage. Annual weeds compete with grapevines for soil nutrients and water. In newly planted vineyards, uncontrolled weeds can easily outgrow vines and deprive them of light. Even in established vineyards, thick stands of annual weeds can reduce vine vigor and yield. Weeds such as marestail, Conyza canadensis, which grow up into the vine canopy, can interfere with harvest and other field operations. Annual weeds, especially winter annuals, also serve as hosts for leafhopper, cutworm, and possibly other grape pests.
Life Cycle. Annual weeds reproduce from seeds and complete their life cycles within one year. Summer annuals overwinter as seeds; they germinate in the spring, flower and produce seeds in the summer or fall, and die in the fall. Winter annuals, which germinate in the fall, overwinter in a vegetative stage. They flower, produce seeds, and die in the spring.
Control. Annual weeds in vineyards are controlled using preemergence herbicides, postemergence herbicides, and mechanical cultivation. Mowing is also used, but it is not very effective. Many weeds reform their flowers close to the ground after being mowed. Nonbearing vineyards, because of their susceptibility to herbicide injury, are sometimes hand weeded.
Oxyfluorfen, oryzalin, simazine, norflurazon, and diuron are the major preemergence herbicides used. Napropamide, pronamide, trifluralin, and pendimethalin are used on smaller acreages. Two preemergence herbicides that are active on different weed spectrums, oxyfluorfen and oryzalin, are often tank mixed. Preemergence herbicides are applied in the late fall or early spring.
Glyphosate and paraquat are the only two widely used postemergence herbicides, but 2,4-D, glufosinate, and sethoxydim also have limited use. Glyphosate is applied in the spring before budbreak and in the fall, but not during the growing season when sucker growth is present on the vines. Glyphosate often is tank mixed with preemergence herbicides. Paraquat is applied during the summer, because warm weather improves its effectiveness.
Mechanical cultivation is done throughout the growing season. If no herbicides are used, mechanical cultivation is usually done at least six times annually to achieve reasonable weed control. Some Pacific Northwest growers use mechanical weed control because they fear herbicides will damage the vines.
Cultivation, however, has other disadvantages. It is relatively slow, it can damage the vines and trellis, it requires more labor and equipment, and it generates dust that can increase mite problems. Also, it is very difficult to cultivate between vines, so there are more weed "escapes" occur than with an herbicide program.
Marestail; puncturevine, Tribulus terrestris; green foxtail, Setaria viridis; sandbur, Cenchrus longispinus; bursage, Ambrosia acanthicarpa; kochia, Kochia scoparia; and Russian thistle, Salsola iberica, are some of the most difficult annual weeds to control in Pacific Northwest vineyards.
Perennial Weeds (Idaho, Oregon, and
Washington wine grapes, juice grapes)
Damage. Perennial weeds compete with grapevines for soil nutrients, water, and sometimes, light. They reduce vine vigor and yield, and interfere with harvest and other field operations. Some perennial weeds are very difficult to control. Field bindweed, Convolvulus arvensis, is a particular problem in Pacific Northwest grapes. Its twining, creeping stems, which grow up vines and trellises and over the canopy, help the plant compete against grapevines for light.
Life Cycle. Perennial weeds regenerate each year from roots, crowns, shoots, and other structures. Some perennials can reproduce only sexually, but others can reproduce both sexually and asexually. Breaking the underground plant structures of some perennial weeds with tillage will cause them to spread because new plants will sprout from each individual piece of the broken plant structure.
Control. Preemergence herbicides can stop perennial weed seeds from germinating. The only preemergence herbicide effective against field bindweed is trifluralin. Trifluralin requires mechanical incorporation, which is expensive and difficult to do in the vine row without leaving untreated areas.
After emergence, spot treatments or broadcast applications of glyphosate can kill most perennial weeds effectively. Nontranslocated contact herbicides such as paraquat and glufosinate will kill perennial weeds only after repeated applications.
Mechanical cultivation is ineffective in controlling perennial weeds, and may even cause them to spread. Hand weeding is only partially effective against species such as field bindweed, whose root systems can penetrate up to 10 feet deep.
Other perennial weeds that can cause problems in Northwest vineyards are Canada thistle, Cirsium arvense; bermudagrass, Cynodon dactylon, and quackgrass, Agropyron repens.
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